The present invention is a low-contaminate work surface for processing semiconductor grade silicon. The work surface is comprised of a parallel array of silicon elements forming a planar surface. The silicon elements are of comparable purity with the semiconductor grade silicon to be process, thus minimizing contact contamination. In an additional embodiment of the present invention, the low-contaminate work surface is part of a work station which provides for initial screening and sizing of the semiconductor grade silicon being processed.
The production of high density integrated circuits requires wafers of monocrystalline silicon of high purity. Transitional metal impurities including, among others, copper, gold, iron, cobalt, nickel, chromium, tantalum, zinc, and tungsten, and impurities such as carbon, boron, and phosphorous are particularly harmful. These impurities, even in small quantities, introduce defect sites in semiconductor material which can ultimately result in degraded device performance, and limited circuit density.
Typically, a polycrystalline silicon of high purity is formed by chemical vapor deposition of a high purity chlorosilane gas onto a heated substrate. The resulting product is rods of polycrystalline silicon. The polycrystalline must be further processed to produce a monocrystalline silicon from which silicon wafers can be cut.
A significant portion of the monocrystalline silicon required by the semiconductor industry is produced by the well known process first described by Czochralski. In a typical Czochralski type process, silicon pieces are melted in an appropriate vessel and a silicon seed crystal is used to draw a monocrystalline rod of semiconductor grade silicon from the melt. Control of this crystal growth process requires that the silicon pieces added to the melt containing vessel be within a defined size range. Therefore, it is necessary that the polycrystalline rods formed during the chemical vaporization deposition process be broken into pieces, and that these pieces be sorted into appropriate size distributions.
Belk, U.S. Pat. No. 4,857,173, describes an apparatus and process for separating silicon seed particles from silicon dust and large, heavy silicon particles. The apparatus is a vertically oriented column having an inert gas flowing upward through the column. A mixture of various size particles is dispensed into the central portion of the column such that small dust particles and product size particles are entrained in the flowing gas and heavier particles fall to a receiver at the bottom of the column. The product size particles are captured in a receptacle near the top of the column. Belk states that, when a high purity mixture of particles is classified with the apparatus or by the method of the invention, it is preferable to use a noncontaminating substance for contact with the various sized particles that are classified.
Dumler et al., Co-Pending U.S. Pat. No. 07/513,409, describes a device for separating semiconductor grade silicon pieces by size. The described device is a cylindrical screen, with contact surface of semiconductor grade silicon, rotationally contacted with a means for rotating the cylindrical screen. The screening device minimizes surface contamination of the silicon pieces.
The inventors have found that the work surface on which silicon rods are processed into pieces is also a source of contamination of the silicon pieces. Therefore, what is described herein is a low-contaminate work surface for processing semiconductor grade silicon. The work surface comprises a parallel array of silicon elements forming a planar surface. The silicon elements are formed from silicon of comparable purity to that of the semiconductor silicon to be process so as to avoid contact contamination of the processed silicon. The silicon elements may be arrange so as to provide an initial size screening of the processed silicon.